1. Technical Field
The present disclosure relates to a positioner for controlling an opening of a valve, and more particularly, to a double nozzle type positioner.
2. Related Art
A control valve represents a valve capable of adjusting an opening of a valve according to an external control signal and is an essential part giving a serious influence on efficiency and performance of various processing automations in power plants, water treatments, petrochemical industries or the like. In particular, the control valve is essential for controlling high-temperature and high-pressure fluid flowing in various tubes installed at a large plant such as a power plant, and the control valve not only controls a flow rate, pressure and flow direction of fluid but also performs main functions such as opening/closing of a channel, throttling, checking, overpressure protection or the like.
Generally, a control signal uses a current of 4 to 20 mA in order to prevent signal distortion against various kinds of noise generated at a site, and a pneumatic pressure is used as an auxiliary power source for operating a valve.
The control valve briefly includes a valve body, an actuator and a positioner, and is classified into a linear type valve and a rotary type valve depending on whether the valve is operated linearly or rotationally. The actuator plays a role of driving the valve by using a pneumatic pressure serving as an auxiliary power source to push a stem connected to the valve body or generate a rotation torque. Therefore, the specification of the actuator is determined according to power and moving distance (or, rotation angle) required for driving the valve body. The positioner is a control unit for measuring a plug opening by means of a sensor connected to the valve body (accurately, the stem), comparing the plug opening with a command signal (4 to 20 mA) input from the outside, and controlling a pneumatic pressure supplied to the actuator until the opening of the valve becomes identical to the command signal.
FIG. 1 shows an existing positioner including a single nozzle, a single flapper and a single pilot valve. The positioner includes a single flapper 1, a single nozzle 2 and a single pilot valve 4. In addition, the positioner further includes an orifice 3 for maintaining a constant pressure of the nozzle 2. An input portion of the pilot valve 4 is connected to the feed pressure, an output portion is connected to the actuator 5. At the output portion of the pilot valve 4, the actuator is operated according to a pneumatic pressure supplied to the actuator 5.
The positioner however has a drawback since the pneumatic pressure supplied to the output of the pilot valve 4, namely the actuator 5, is seriously influenced by external environments.
FIG. 2 shows an existing arrangement where two positioners are coupled to a single actuator in order to overcome the drawback of the positioner of FIG. 1. In this arrangement, two flappers 1, two nozzles 2, two orifices 3 and two pilot valves 4 are connected to a single actuator 5. For example, the output of one pilot valve 4 is connected to move the actuator 5 upwards, and the output of the other pilot valve 4 is connected to move the actuator 5 downwards. Each of these sets has the same configuration as the positioner of FIG. 1.
In this arrangement, the actuator 5 is operated due to a difference between the output of one pilot valve 4 and the output of the other pilot valve 4. In general, two pilot valves receive external influences similarly, and thus the external influences are offset at the difference in two outputs. As a result, this arrangement is not seriously influenced by external environments.
However, the assembling process for coupling positioners with an actuator becomes more cumbersome, and two positioners should be controlled at the same time. In other words, the actuator may malfunction due to timing between control signals for controlling two positioners and any error possibly existing at the outputs of the positioners.